1. Field
The invention is in the field of producing titanium powder.
2. State of the Art
There is a growing demand for high purity (99%), low oxygen content (less than 0.25%) titanium powder. Such powder, among other uses, may be used in metal injection molding processes where a metal powder is molded under pressure to about 95% density and is then fired at high temperatures in a reduced atmosphere to finish the product.
Hydriding and dehydriding processes are known. The hydriding of a metal to a metal hydride allows for production of fine powder. The metal hydride is generally much more frangible than the metal and can be more easily milled or otherwise crushed. After powdering, the hydride has to be dehydrided to provide a metal powder, particularly a high purity metal powder product. Generally the hydride powder is placed in a crucible and heated to a high temperature to cause dehydriding. With titanium, however, when heated to the high temperature, the powder tends to sinter into a mass which is no longer powder, and at elevated temperatures, the product reacts with oxygen. The oxidation is a problem with powder because of the large surface area of the powder particles. While high purity, low oxygen titanium powder is currently available, it is relatively expensive. The relatively high cost makes its use impractical for many products.
According to the invention, the sintering and oxidizing of a hydrided metal powder during dehydriding is reduced by arranging the powder for heating in a relatively thin layer of not greater than about one-quarter inch, slowly heating the powder during dehydriding over a period of several days, and providing a partial vacuum over the powder during heating. In accordance with the invention, a high purity, low oxygen content titanium powder is produced from a hydrided titanium powder by a dehydriding process where the powder is dehydrided in thin layers, one quarter inch or less thick, and the temperature is increased slowly as the hydrogen is released. The heating takes place in a reduced pressure atmosphere to draw out the hydrogen, and after dehydriding, upon cooling, the powder is kept in an inert atmosphere for any required further pulverization and packaging.
The process may start with a hydrided titanium powder of a desired percentage of particles less than a desired size. For a commercially desirable product useful for metal injection molding as well as other uses, it is desirable that the hydrided powder have about ninety percent or more of particles less than twenty-five microns in size with most particles in the ten to fifteen micron size range. This provides a powder of less than about 99.5% xe2x88x92325 mesh. This hydrided powder must be dehydrided. For dehydriding, the hydrided powder is spread on a tray in a relatively thin layer in the range of from one-eighth to one-quarter inch thick. The powder is heated to between about 450xc2x0 C. and 500xc2x0 C., usually relatively quickly over a period of about six hours. From there, heating continues slowly, under negative pressure (a partial vacuum) over a four to five day period until the powder reaches a temperature in the range of 650xc2x0 C. to 700xc2x0 C. This dehydrides the hydrided powder and will normally reduce the hydrogen content of the powder to less than 0.1% with minimal sintering of the powder. Faster temperature increases, particularly with increases to the final temperature range in four to five hours, generally results in substantial sintering of the powder, which is undesirable. Further, placing the powder in a crucible which generally results in a thickness of powder greater than one-quarter inch during heating also generally results in substantial sintering of the powder. The combination of the invention of the relatively thin layer of powder on a tray and slowly increasing the temperature over a period of several days, substantially reduces the sintering of the powder, however, some sintering still occurs.
The dehydrided powder is cooled, usually to about room temperature, maintaining the negative pressure atmosphere. As the vacuum is released, an inert atmosphere, such as an argon atmosphere, is introduced and maintained during further processing and packaging of the titanium powder. When cooled to around room temperature, the powder is transferred from the tray to a ball mill or hammer mill for further pulverization as needed to break up any sintered particles to maintain the desired particle size. The powder is then transferred to a glove box for screening to ensure a desired size specification, such as 99.5% xe2x88x92325 mesh, and is packaged in the inert atmosphere in polybags or polybottles and then in cans to maintain the inert atmosphere during shipping and storage of the powder.
The starting hydrided titanium powder is produced by hydriding titanium sponge and then crushing the hydrided titanium to the desired percentage of desired size particles by any desired method such as in a ball mill or hammer mill.